It does not affect wifi signal much but I wonder what kind of weight could this device support?

My cat is only 5 months old, I assume she will get heavier one day...

My cats don't sit on it because it's not in a location that's conducive to cat lounging. In your case I wouldn't worry about the weight or the signal. My only concern would be that the Airport would overheat and fail prematurely with a cat sleeping on on it blocking the heat dissipation through the top. I already had an Airport die of excess heat and that was without any cats sitting on it. Apple, however was good about replacing it.